1. Field of the Invention
The present invention relates to catalyst for a fuel reforming reaction in a fuel cell, and fuel reforming apparatus and method using the same, particularly to catalyst for a partial oxidation reforming reaction of fuel and fuel reforming apparatus and method using the same, which are suitable for a fuel reforming system.
2. Description of the Related Art
Since a fuel cell exhibits a very excellent efficiency and exhausts little pollutional material, compared to an internal-combustion engine, researches on the fuel cell have been constantly carried out. However, a fuel reforming system (fuel processor) capable of stably supplying hydrogen-rich gas, which is fuel required for the fuel cell, is necessary to be developed.
In the fuel reforming system, main reactions, particularly when using methane are as follow:CH4+H2O→CO+3H2, ΔH298=206 kJ/mol  <Chemical formula 1>CH4+1/2O2→CO+2H2, ΔH298=−36 kJ/mol  <Chemical formula 2>
The chemical formula 1 is a steam reforming (SR) reaction, which is widely commercialized. This reaction exhibits relatively high yield of hydrogen and stability. However, since the reaction is highly endothermic, large amounts of heat should be supplied from the outside. In addition, the apparatus becomes large because conversion rate is limited and reaction rate is slow.
The chemical formula 2 is a partial oxidation (POX) reaction of methane, which is fuel for fuel cell. Since this reaction is exothermic, heat supply is minimized and the reaction rate is fast compared to the steam reforming reaction. Further, the size of apparatus becomes small and the conversion rate becomes high. However, there is a possibility of explosion and the yield of hydrogen is reduced when using the air as oxygen source.
As a reaction occurring in the fuel reforming system in addition to the above reactions, there is an auto thermal reforming (ATR) [or partial oxidation reforming (POR)] reaction in which the steam reforming reaction and the partial oxidation reaction occurs simultaneously.
Recently, in order to develop a small fuel reforming system required for a mobile fuel cell, methods using a partial oxidation reforming reaction, which has reasonably fast reaction rate and response, have been researched.
Main reaction paths suggested as mechanism of the partial oxidation reforming reaction are as follow:
Chemical formulas 3 to 5 show each reaction stage of the partial oxidation reforming reaction.CH4+2O2→CO2+2H2O(first stage)  <Chemical formula 3>CH4+H2O→CO+3H2(second stage)  <Chemical formula 4>CH4+CO2→2CO+2H2(third stage)  <Chemical formula 5>
The chemical formula 3 shows that methane reacts with O2 and thus is converted completely into CO2 and H2O in a short time.
The chemical formulas 4 and 5 show that CO2 and H2O produced in the chemical formula 3 reacts with the remaining methane [Dissanayake, D., Rosynek, M. P. and Lunsford, J. H., J. Phys. Chem., 97, 3644-3646(1993); Prettre, M., Eichner, Ch. and Perrin, M., Trans. Faaday Soc., 43,335-340 (1946)].
In the partial oxidation reforming reaction, a highly exothermic reaction occurs in a moment at the early stage and then the reforming reaction, which is endothermic, occurs slowly.
Meanwhile, catalyst in the prior reforming reaction system has been in the form of powder. Further, catalyst in the form of powder is coated to a structure which is compression-molded in the form of pellet or monolith [Wang Lai Yoon, et al., HWAHAK KONGHAK, 41(3), 389-396 (2003); Dong Ju Moon, et al., Appl. Catal. A, 215, 1-9 (2001); Ahmedm K., et al., Solid State Ionics, 152-153, 485-492 (2002)].
However, in the partial oxidation reforming reactions of fuel as described above, when catalyst in the form of powder, etc. is used, pressure drop which occurs in the powder catalyst bed in operating a fuel reforming system, should be considered and controlled. When catalyst in the form of pellet or monolith is used, it is difficult to control heat of reaction when the reaction occurs.
Accordingly, the prior fuel reforming system has the problems that it is difficult to simplify a reactor, to obtain a fast starting time and to carry out a stable operation.